Conventionally, planetary gear reducers have been installed in power transmission mechanisms for use in extracting power from a power source such as an aircraft gas turbine engine. Such a planetary gear reducer includes: a sun gear having external teeth; a plurality of planet gears having external teeth and meshing with the sun gear; a shared planet carrier supporting the rotational axis of each planet gear and positioning the plurality of planet gears relative to each other; and a ring gear having internal teeth and meshing with the plurality of planet gears (see Patent Literature 1, for example). In the planetary gear reducer having such a configuration, power that is generated from a power source is, for example, inputted into the sun gear first, then transmitted to the planet gears, and thereafter the power can be extracted from the planet gears as two different kinds of outputs. Specifically, the power transmitted to the planet gears can be extracted as rotative power of the ring gear that is derived from rotation of the planet gears and as rotative power of the planet carrier that is derived from revolution of the planet gears around the sun gear.